Spot welding generally joins together similar metal members with each other. However, if it is applicable to joining of dissimilar metal members (dissimilar material joint product), for example, joining of an iron-based material (hereinafter simply referred to as a “steel product”) and an aluminum-based material (a generic term for pure aluminum and aluminum alloys; hereinafter simply referred to as an “aluminum material”), this can greatly contribute to weight saving and the like.
However, in joining together a steel product and an aluminum materials a brittle intermetallic compound is readily produced at the joint part and it is very difficult to obtain a joint part having reliably high strength (bonding strength). Accordingly, joining for such a dissimilar material joint product (joint product of dissimilar metal members) has been conventionally performed using a bolt, a rivet or the like, but this has a problem in the coupling joint reliability, airtightness, cost and the like.
To solve such a problem, many studies have been heretofore made on a spot welding method for these dissimilar material joint products. For example, a method of inserting an aluminum-steel clad material between an aluminum material and a steel product has been proposed (see, Patent Documents 1 and 2). Also, a method of plating or inserting a metal having a low melting point on the steel product side has been proposed (see, Patent Documents 3, 4 and 5). Furthermore, there have been proposed, for example, a method of sandwiching an insulator particle between an aluminum material and a steel product (see, Patent Document 6) and a method of previously creating irregularities on a member (see, Patent Document 7).
However, all of these methods involve not a simple spot welding but a spot welding in multiple layers, require a separate step such as plating or working, raise an issue that new equipment must be incorporated into the ousting welding line, or incur a rise of the welding cost. Furthermore, these methods all have many problems in view of operation, such as severe limitations on the welding conditions.
Patent Documents 8 to 11 have proposed a method of applying a film such as plating to steel by a different technique. These patent publications are aiming at reducing the interface reaction layer or creating irregularities on the interface by forming a highly general-purpose film having a low melting point between steel and an aluminum material, because the previously plated steel or aluminum material allows for direct welding, and each demonstrates that higher strength than that in direct joining of steel with an aluminum material is obtained. Also, depending on the kind of plating, dissimilar metal contact corrosion, which occurs between steel and an aluminum material, can be suppressed. The film formed is Mg in Patent Document 8, a film lower in the melting point than an aluminum material in Patent Document 9, a film lower in the melting point than steel in Patent Document 10, and a film having a melting point which is not less than a temperature 300° C. lower than the melting point of an aluminum material and is a temperature lower than the melting point of an aluminum material in Patent Document 11.
However, in Patent Document 8, the strength is 80 kgf and is insufficient, though the breaking is a base material rupture. In Patent Document 9, sufficiently high tensile shear strength is obtained but a nugget is not formed. Merely by an anchor effect owing to such microscopic bonding, tensile shear strength may be ensured but cross tensile strength (peel strength) cannot be maintained, and the usage is limited to special cases involving only tensile shear stress.
In Patent Document 10, a resistive element needs to be further inserted and this not only raises the above-described issue that new equipment must be incorporated into the existing welding line, but also incurs a rise in the cost. In Patent Document 11, although base material rupture is obtained with respect to the cross tensile strength, an interface reaction layer is not formed and this is described as an action of realizing high strength. Certainly, the interface reaction layer which is an intermetallic compound of steel and aluminum is brittle, but according to the present inventors, it is found that if the interface reaction layer is not present at all as in Patent Document 10, an adherence layer by interdiffusion is in turn not formed and low bonding strength results. Accordingly, similarly to Patent Document 9, merely by microscopic bonding, sufficiently high cross tensile strength cannot be maintained, and the demand for a higher-strength joint product cannot be responded to.
The present inventors have continuously made studies mainly for optimizing the structure of the interface reaction layer at the joint part formed by spot welding and found that a dissimilar material joint product having high bonding strength can be produced by controlling the thickness, area and structure of the interface reaction layer.
In Patent Documents 12 to 15, the present inventors have proposed a technique where a peel strength of 0.9 kN/spot or more can be obtained by controlling the thickness and melting point of plating film and further controlling the nugget diameter and the thickness of interface reaction layer.    Patent Document 1: JP-A-6-63763    Patent Document 2: JP-A-7-178563    Patent Document 3: JP-A-4-251676    Patent Document 4: JP-A-7-24581    Patent Document 5: JP-A-4-143083    Patent Document 6: JP-A-5-228643    Patent Document 7: JP-A-9-174249    Patent Document 8: JP-A-4-143083    Patent Document 9: JP-A-4-251676    Patent Document 10: JP-A-7-24581    Patent Document 11: JP-A-7-178565    Patent Document 12: JP-A-2005-305504    Patent Document 13: JP-A-2005-152958    Patent Document 14: JP-A-2005-152959    Patent Document 15: JP-A-2006-167801